libxc: Handle failed xc_get_hvm_param in domain save

libxc: Handle failed xc_get_hvm_param in domain save

The domain save code will read an HVM param, and if it's not zero,
make an entry for it.  However, if the hypercall fails for any reason,
the data may not be written, and the value for the previous parameter
may be written in the save file as the parameter that failed.

Initialize the value to zero before each hypercall, so that in case of
a failure, no value will be written.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>